1. Field of the Invention
The present invention relates to a support for a mask used in X-ray lithography processing for preparing integrated circuits, and a process for preparing the support.
2. Related Background Art
Recent semiconductor integrated circuits are demanded to be made small in size and made highly integrated, and a method of forming finer patterns is required in the preparation process therefor. Accordingly, it has become popular to utilize the techniques of X-ray lithography.
In general, a mask used in X-ray lithography processing comprises a support frame having the shape of a ring or the like and a film member having X-ray transmitting portions and X-ray non-transmitting portions. The film member covers the opening of the support frame. In usual cases, the X-ray non-transmitting portions are formed of an X-ray opaque or absorbing material having a geometrical pattern and provided on a mask support film (membrane) that constitute a base for the film member, and the X-ray transmitting portions are formed of the mask support film itself corresponding to the part on which the X-ray opaque or absorbing material is not provided.
The X-ray opaque or absorbing material must be formed on the mask support film with a submicroscopic size, with a good precision and with good reproducibility, and also the mask support film is required to have dimensional stability. The mask support film, which covers the support frame with a considerable tension, is also required to have mechanical strength.
Since it is required to transfer mask patterns to a semiconductor wafer with a submicroscopic size and precision without causing any deformation even by heat due to the absorption of the incidence of X-rays, a mask support film comprising a ceramic material has been proposed so that the above requirements can be fulfilled.
The support film comprising a ceramic material is prepared by forming a film on a substrate by a gaseous phase method such as physical vapor deposition (PVD), chemical vapor deposition (CVD) and molecular beam deposition (MB), followed by removal of the substrate.
Preparation of the mask support film comprising SiC, which is a preferred ceramic in view of the mechanical strength or less thermal deformation, is carried out according to a gaseous phase film deposition method typified by the method described in Japanese Patent Publication No. 53-24785, in which a reactive ion-plating method is employed to produce an SiC film on a silicon substrate in an atmosphere of a hydrocarbon having 1 or 2 carbon atoms.
However, such a preparation method according to the gaseous phase film deposition has a difficulty in mass production, resulting in expensiveness. Because a gaseous phase film deposition apparatus is very expensive, the number of the films that can be formed at one time is limited, and the film deposition rate is not high. Besides, it has been not suitable to mass production also for the following reasons:
(a) Since the properties of films to be formed depend on various parameters such as substrate temperature, gas concentration and gas purity which are dependent on each other, conditions for film deposition must be set with complicacy and also maintained with difficulty. PA1 (b) The operation to remove the substrate by etching or the like after film deposition is essential, and such an operation is cumbersome. PA1 (1) Because of the crystal particle diameter that comes to be from .about.1 .mu.m to several .mu.m, the visible light or near infrared light used for alignment undergoes such large scattering that the transmittance may be lowered and the S/N ratio of the alignment light signal may be lowered. PA1 (2) Since the irregularities indicate a thickness non-uniformity, they may cause non-uniformity in the intensity of transmitting X-rays. PA1 (3) Because of lack of smoothness of the surface, there may be caused deterioration of precision, non-uniformity in the thickness of X-ray opaque or absorbing materials, and adhesion failure of X-ray opaque or absorbing material patterns, when fine patterns are formed.
There is also the problem that a film having irregularities the same as an order of its thickness is formed because, in the mask support film formed by the gaseous phase film deposition method, crystals that grow from various points on the substrate collide with each other and the axis or lattice plane of the crystals appear almost randomly.
The irregularities on the surface of the support film bring about the following undesirable results.
Moreover, the film is so thin and so hard that it can only be flattened with difficulty. Thus, the mask support film having surface roughness is not suitable to the formation of X-ray opaque or absorbing materials with good precision, and the mask support film formed by the gaseous phase method has not necessarily had satisfactory performances. In addition, an adhesive is always used to fix the above ceramic film to a support member, but adhesives for practical use are organic polymers. Accordingly, even when a heat-resistant adhesive such as polyimide is used, the tension applied to the film is loosened to make it difficult to retain the submicron patterns, because of the large thermal expansion coefficient (about 100 times the ceramics) of such heat-resistant adhesive.